The present invention relates to a tracking servo device used in a recorded information reading device.
In a device for reading recorded information from a disc-like recording medium such as a video disc, a digital audio disc, or the like, a tracking servo system is employed with which it is ensured that the recorded track is always accurately traced by an information reading point in spite of eccentricity of the disc, etc. In some reading devices, an information search function (random access function) is additionally provided so that the information reading point can be driven in the direction substantially perpendicular to the track, that is, in the radial direction of the disc, to thereby perform track jumping.
FIG. 1 is a schematic block diagram showing such a conventional reading device. As shown in this drawing, there is provided a tracking error signal generating circuit 1 for generating a tracking error signal indicative of an amount of positional shift in the radial direction of the disc between the information reading point of a pick-up and the recording track of the disc. The tracking error signal generation circuit 1 has a well-known structure, for example, it can be implemented by a pair of light-detecting elements receiving a pair of respective light spots which are reflected from the recorded surface of the disc and a differential amplifier for producing a signal representing the difference between the outputs of the two light-detecting elements. The tracking error signal is supplied to a driver amplifier 4, constituted by an operational amplifier OP.sub.1, through an equalizer 2 and a loop switch 3. A tracking actuator, driven by the output of the driver amplifier 4, effects displacement of the information reading point in the radial direction of the disc. In this example, the actuator is constituted by a coil 5 and a tracking mirror 6 rotatably driven by the coil 5. The current flowing in the coil 5 is detected by a resistor R.sub.1, and the detected output is fed back to the driver amplifier 4 through a feedback resistor R.sub.2 to thereby stabilize the system.
The arrangement as described above forms part of the tracking servo system. The tracking servo system also includes a jump operation control system (described below in more detail). Accordingly, the reading device can perform random access, that is, recorded information searching, on the disc.
The jump operation control system includes a controller 7 receives a tracking error signal A and a jump instruction signal B and produces in response a control signal C for controlling the on/off operation of the loop switch 3 and a drive/jump brake signal D which is superimposed on an output of the loop switch 3.
FIG. 2 is a waveform diagram used for explaining the operation of the device of FIG. 1. In FIG. 2, A to D indicate waveforms of the signals A to D, respectively, at the indicated circuit points in FIG. 1. When the information reading point is moved in the radial direction of the disc, the tracking error signal A is generally a sinusoidal wave signal having a period corresponding to the distance between adjacent tracks.
The zero-crossing points a and b of the signal A occur at centers of adjacent tracks. Where jumping of one track is performed (from the track of the point a to the track of the point b), when the jump instruction signal B is applied to the controller 7 when the information reading point at the point a, at which time the loop switch 3 is in its closed state in response to a high-level control signal C so that the servo loop is closed, the level of the control signal C is made low to open the loop switch 3 and thus open the servo loop. At the same time, a jump drive signal P, as shown by D in FIG. 2, is produced by the controller 7 and applied to the coil 5 (constituting the actuator) through the driver amplifier 4.
Accordingly, the tracking mirror 6 is rotated by the drive coil 5 to cause the information reading point (light spot) to move in the radial direction of the disc, that is, in the direction perpendicular to the track. The level of the tracking error signal A becomes zero when the information reading point reaches a position substantially midway between two recording tracks and becomes zero again when it reaches the center of the next recording track at b. The level of the control signal C produced by the controller 7 changes from low to high at b to thereby close the servo loop. At the same time, a jump brake signal Q, as shown by D in FIG. 2, is produced by the controller 7 with which the movement of the actuator is braked. Thus, the information reading point is jumped to the adjacent track to thereby complete one track jumping operation.
The thus-arranged conventional device has a drawback that, in order to accurately perform the jumping operation, it is necessary to very precisely control the various waveforms, timing points, peak values, total energies, etc., of the jumping drive and brake signals P and Q shown in FIG. 2. Also, it is difficult to perform accurate jumping over several recording tracks, and thus the jumping operation cannot always be stably performed. Further, since the jumping operation is performed with the servo loop opened, there may be some cases where the number of tracks jumped varies depending on the degree of eccentricity of the disc.